Lasers are used in a variety of machining applications. In particular, continuous wave (CW) and long pulse width lasers have been used to cut, drill, surface modify, and mark target materials. Energy from an impinging laser beam or pulse typically beats a target material from a solid phase through a liquid phase to a vapor phase causing expansion and expulsion of target material.
Problems arise in micromachining, however, because laser micromachined materials typically require high precision and quality. For CW and long pulse width lasers, the distance over which the heat from the laser pulse is dispersed is greater than the absorption length of the laser. As a result, partial re-solidification of the melt zone can occur causing the formation of craters and heat-affected zones. Other problems include surface contamination and shock wave damage to adjacent material.
One solution is to use ultrashort pulse width lasers, such as, for example, pulses generated by femtosecond and picosecond lasers. Because ultrashort pulse width lasers deposit energy into a target material in a very short time interval, the distance over which the heat due to the laser is dispersed is less than the absorption length of the laser. Thus, material is removed before energy loss due to thermal diffusion can occur. U.S. Pat. No. 5,656,186 (“the '186 patent”), reissued as U.S. RE Pat. No. 37,585, discloses a method for ablating a feature smaller than the laser spot size using pulse widths less than one nanosecond and pulse energies on the order of 1 micro joule (μJ). The '186 patent discloses plotting the relationship of the fluence threshold at which breakdown occurs versus the laser beam pulse width to determine a transition point at which the ablation threshold is highly dependent on pulse width. According to the '186 patent, when operating near this transition point a bulk of the pulse energy ionizes the target material to cause ablation. Because the bulk of the pulse energy ionizes the target material, micromachining at the fluence levels disclosed in the '186 patent may result in unacceptable precision and quality of the laser processed material.
Thus, there is a need to overcome these and other problems of the prior art and to provide methods and apparatus for laser micromachining. The present invention, as illustrated in the following description, is directed to solving one or more of the problems set forth above.